Apparatus and method for unidirectionally solidifying high temperature material

ABSTRACT

An apparatus and method for unidirectionally solidifying high temperature materials comprising a graphite resistance furnace having a heat exchanger vertically positioned in the chamber thereof. The heat exchanger has inlet and exit means for the passage of an inert coolant gas therethrough. In operation, a crucible is loaded with a seed and the ceramic material to be melted and positioned in the interior of the chamber of the furnace in contact with the heat exchanger. The temperature of the melt is raised to about 50* C above its melting temperature. The melt temperature is slowly decreased with a correspondingly increase in the flow of the inert coolant gas through the heat exchanger thereby unidirectionally solidifying the material to produce a single crystal.

:11 Elatea Patent [151 3,653,432

Sehmid et al. [451 Apr. 3, 1972 54] APPARATUS AND METHOD FOR 3,414,661 12/1968 Reed ..263/40 R UNIDIRECTIONALLY SOLIDIFYING HIGH TEMPERATURE MATERIAL Primary Examiner-John J. Camby Attorney-Harry M. Saragovitz, Edward J. Kelly and Herbert Berl [5 7] ABSTRACT An apparatus and method for unidirectionally solidifying high temperature materials comprising a graphite resistance furnace having a heat exchanger vertically positioned in the chamber thereof. The heat exchanger has inlet and exit means for the passage of an inert coolant gas therethrough. In operation, a crucible is loaded with a seed and the ceramic material to be melted and positioned in the interior of the chamber of the furnace in contact with the heat exchanger. The temperature of the melt is raised to'about 50 C above its melting temperature. The melt temperature is slowly decreased with a correspondingly increase in the flow of the inert coolant gas through the heat exchanger thereby unidirectionally solidifying the material to produce a single crystal.

14 Claims, 2 Drawing Figures il jj [72] Inventors: Frederick Schmid, Marblehead; Dennis J.

Viechnicki, Wellesley, both of Mass.

[73] Assignee: The United States of America as represented by the Secretary of the 'Army [22] Filed: Sept. 1, 1970 [21] Appl. No.: 68,803

[52] US. Cl ..l65/61, 263/40 [51] Int. Cl ..F25b 29/00 [58] Field of Search ..263/11, 14,40; 165/30, 61

[56] References Cited UNITED STATES PATENTS 3,468,523 9/1969 Dix ..263/40 R Patented April 4, 1972 INVENTOR;

APPARATUS AND METHOD FOR UNIDIIRECTIONALLY SOLllDIFYING HIGH TEMPERATURE MATERIAL The invention described herein may be manufactured, used, and licensed by or for the Government for governmental purposes without the payment to us of any royalty thereon.

This invention relates to an apparatus and method for the unidirectionally solidification of the ceramic material to obtain a single crystal or dense polyphasic shape.

It is an object of this invention to provide and disclose a system for the unidirectional solidification of high temperature ceramic material to produce a single crystal or dense polyphasic shape.

It is a further object of this invention to provide and disclose a novel heat exchanger component of a system for the unidirectional solidification of high temperature ceramic material to produce a single crystal or dense polyphasic shape.

It is a further object of this invention to provide and disclose a method for the unidirectional solidification of high temperature ceramic material to produce a single crystal or dense polyphasic shape.

It is a further object of this invention to provide and disclose a controllable growth method for the unidirectional solidification of high temperature ceramic material to produce a single crystal or dense polyphasic shape.

It is a further object of this invention to provide and disclose a controllable solid state cooling cycle method for the unidirectional solidification of high temperature ceramic material to produce a single crystal or dense polyphasic shape.

Other objects and a fuller understanding of the invention may be had by referring to the following description and claims taken in conjunction with the accompanying drawing in which:

FIG. 1 shows a schematic of the gradient furnace system.

FIG. 2 shows a prospective view of the heat exchanger.

Referring now to the drawing, a specific example of a system of the type according to the present invention comprises furnace housing 10. The housing is suitable, contoured to form chamber 11. The heating means 12 may be constructed of any suitable refractory material, e.g., graphite. Heating means are positioned in chamber 11. Any conventional heating means, e.g., electrical resistance means, may be utilized. The heating means are connected to any suitable electrical outlet, not shown. Vacuum pump 14 is utilized to apply a low vacuum to the chamber of the furnace through conduit means 16. The temperature of the furnace is indicated by any conventional means, e.g., pyrometer 18.

The heat exchanger comprises solid cylindrical base segment and upper reduced hollow cylindrical segment 22. Base segment 20 may be constructed of any suitable material, e.g., molybdenum. The top portion of segment 22 forms flat surface 24. The upper portion of segment 22 is composed of tungsten and designated 26. The lower portion of segment 22 is composed of molybdenum and designated 28. Tungsten is utilized in the upper portion of segment 22 due to its superior high temperature properties. Concentric tubing 30, which has open ends, extends through base segment 20 to a point near flat surface 24 of reduced segment 22. Concentric tubing may be constructed of a material having the ability to withstand high temperatures, oxidation and thermal shock, e.g., molybdenum. Exit bore 32 extends through base segment 20 and connects the interior of reduced segment 22 with the atmosphere.

Thermocouple 33 which is positioned in the interior of reduced segment 22, is connected to any suitable temperature indicator means, e.g., a potentiometer, not shown, by leads 35. Base segment 20 of the heat exchanger is attached to housing 10 by means of bolts 34. The bolts are positioned in bores 36 of base segment 20. Reduced segment 22 of the heat exchanger is inserted through an opening, not shown, of housing B0.

In operation, seed crystal 40 is positioned in refractory metal crucible 38. The crucible is then filled with premelted ceramicmaterial 42. The open end of the crucible is then covered with a sheet of refractory metal 44. A molybdenum metal sheet has been found suitable. The crucible is then positioned in chamber 11 on top of flat surface 24 of the heat exchanger, as shown in FIG. 1. A vacuum is applied and the heating elements are activated raising the temperature to about 50 C. above the melting point of the ceramic material. Concurrently, a coolant inert gas is bled from storage 46 through conduit 48 and into concentric tubing 30. The gas cools the base of crucible 38 and exits through bore 32 to the atmosphere. In the alternative, the gases may be collected, cooled and returned to storage. The temperature of the furnace is slowly decreased with a corresponding increase in the flow coolant gas through the heat exchanger, thereby unidirectionally solidifying the ceramic material to form a single crystal. Surface 24 of reduced segment 22 is polished down to a 600 grit finish. It may also be necessary to polish the exterior bottom of crucible 38 in order that the crucible will fit snugly on the heat exchanger.

EXAMPLE 1 A sapphire seed crystal is positioned in the bottom of the crucible with its a axis parallel to the growth direction. The crucible is filled with the premelted A1 0 to be melted. A refractory metal cover is then placed over the open end of the crucible and the system assembled as shown in FIG. 1. The loaded crucible is positioned in the gradient furnace so that the polished base of the crucible is seated directly on the heat exchanger. The polished metal cover is positioned over the open end of the crucible in order to reflect heat radiation back to the metal surface. After the furnace has been evacuated to 5X10torr, the furnace power is turned on and helium bled into the heat exchanger at a slow flow rate, ca. 4 cubic feet per hour (cfh) in order to prevent oxidation of the refractory metal heat exchanger. The temperature of the furnace is increased at a sufficiently slow rate, ca. ll C/min, to prevent the pressure in the furnace from exceeding 2X10 torr. As the melting point of the ceramic material is reached, the flow of helium into the heat exchanger is increased to, ca. 40 cfh to prevent the melt from boiling over the top of the crucible when melting occurs and also to prevent the seed from melting. The temperature of the melt is then increased to 50 C above the melting point of the A1 0 i.e., 2,l00 C. The helium flow is then decreased, ca. to 8 cfh to permit partial melting of the seed crystal. When solidification commences the solid mucleates on the seed and assumes its crystallographic orien tation. The thermocouple positioned in the heat exchanger registers the temperature of the center of the seed. The temperature of the center of the seed is not allowed to increase above melting point of the material by controlling the flow of helium into the heat exchanger. To start solidification of the ingot, the flow of helium is increased gradually in small increments (ca. 0.5 cfh/min), to about cfh to extract heat from the melt in a controlled fashion so that the solid grows from the seed crystal. The melt temperature is then decreased at a rate 2 C/min by decreasing the furnace power until a temperature 30 C below the melting point of the material is reached. This temperature is maintained while the flow of helium is decreased at a rate of 2 cfh/min until a slow rate, e.g., 4 Cfll, necessary to prevent oxidation of the heat exchanger is reached. The furnace power is then terminated and the single crystal allowed to cool to 50 C. in a period of 16 hours. X-ray analysis of the produce using the Laue back reflection technique verified that a single crystal was obtained, i.e., X- ray photographs of different areas of the crystal were identical. The method was repeated with the c axis of a seed crystal parallel to the growth direction; and the 0" axis of a seed crystal 60 from the growth direction. The resulting single crystal took the orientation of the seed crystal.

EXAMPLE 2 The method of Example 1 was repeated with the following differences. Seed of compound Y Al 0 was oriented so that its crystallographic direction will be parallel to the intended growth direction. The crucible was filled with Y Al powder. The temperature of the gradient furnace was raised to 1,980 C, i.e. 50 C above the melting point of Y Al 0 A single crystal ingot of Y Al 0 was obtained. X-ray analysis of the ingot utilizing the Laue back reflection technique verified that a single crystal was obtained. A Y Al 0 single crystal with a [l growth direction was grown from a [110] oriented seed crystal.

Other ceramic material capable of being unidirectionally solidified in accordance with the present method include MgA 0 Al 0 /ZrO eutectic and Al 0 /Y Al 0 eutectic.

Illustrative, but without limitation, a furnace chamber within the scope of this invention comprises a diameter of 4 inches and a height of 8 inches. An illustrative crucible comprises 1 cm in diameter by 21 cm in height.

It is envisaged that the produced, single, transparent crystals can be utilized in lasers, armor, bearing, etc.

Although we have described our invention with a certain degree of particularity, it is understood that the present disclosure has been made by way of example and that numerous changes may be made in the details of construction and arrangemcnt of parts, and that various ceramic materials may be utilized without departing from the spirit and scope of the invention.

Having described our invention, we claim:

1. A system for the unidirectional solidification of ceramic material comprising a housing suitable contoured to form a chamber, heating means positioned in said chamber, a heat exchanger having a base segment and a reduced hollow upper segment, concentric conduit means extending into the upper segment of the heat exchanger to a point near the top section thereof, exit means connecting the interior of the reduced upper segment of the heat exchanger with the atmosphere; in operation the reduced segment of the heat exchanger is positioned in the interior of the chamber with the base segment on the exterior of the housing, the chamber is heated to a temperature above the melting point of the ceramic material and the material cooled to a single crystal and dense polyphasic shape by the passage of a coolant gas through the heat exchanger.

2. A heat exchanger comprising a base segment and a reduced upper segment, concentric conduit means extending into the upper segment of the heat exchanger near the top section thereof, exit means connecting the interior of the reduced upper segment of the heat exchanger with the atmosphere.

3. A heat exchanger in accordance with claim 2 where the upper portion of the concentric means is constructed of tungsten and the lower segment is constructed of molybdenum.

4. A method for the unidirectional solidification ofa ceramic material to form a single crystal comprising the steps of:

a. seeding the melt ofa ceramic material,

b. melting the ceramic material under vacuum and maintaining the melt at about 50 C above its melting point while unidirectionally cooling the melt by the passage ofa coolant gas against the closed end ofa heat exchanger.

c. unidirectionally solidifying the ceramic material by decreasing the temperature while increasing the flow of coolant gas, and recovering the material.

5. A method in accordance with claim 4 wherein the coolant gas is helium.

6. A method in accordance with claim 4 wherein the ceramic material is selected from the group consisting of A1 0 Y Al 0 MgAl 0 Al 0 /Zr0 and Al 0 /Y Al 0 eutectics.

7. A method in accordance with claim 6 wherein the ceramic material is A1 0 8. A method in accordance with claim 7 wherein the premelt is seeded with its a axis parallel to the growth direction.

9. A method in accordance with claim 7 wherein the premelt is seeded with its c axis 60 from the growth direction.

10. A method in accordance with claim 6 wherein the ceramic material is Y Al 0 11. A method in accordance with claim 10 wherein the premelt is seeded with l 10] growth direction.

12. A method in accordance with claim 6 wherein the ceramic material is MgAl 0 13. A method in accordance with claim 6 wherein the ceramic material is Al 0 /Zr0 eutectic.

14. A method in accordance with claim 6 wherein the ceramic material is Al 0 /Y Al 0 eutectic.

UNITED STATES PATENT OFFICE 0-1050 (5/69) CERTIFICATE OFTCORRECTION PatentNo. 3 65.3 3 Dated 412i 4-, 1972 is J. Viechnicki Inventor(s) Frederick schmid and Den ppears in the above-identified patent It is certified that error a hereby corrected as shown below:

and that said Letters Patent are torr-- Cdlumn 2, line 31, x 10' torr" should read: --5 x 10 Signed and sealed this 19th day of March 1974.

(SEAL) Attest': v

EDWARD M.FLETC IHER,JR. c. MARSHALL DANN Attesting Qf iicer f Commissioner of Patents 

2. A heat exchanger comprising a base segment and a reduced upper segment, concentric conduit means extending into the upper segment of the heat exchanger near the top section thereof, exit means connecting the interior of the reduced upper segment of the heat exchanger with the atmosphere.
 3. A heat exchanger in accordance with claim 2 where the upper portion of the concentric means is constructed of tungsten and the lower segment is constructed of molybdenum.
 4. A method for the unidirectional solidification of a ceramic material to form a single crystal comprising the steps of: a. seeding the melt of a ceramic material, b. melting the ceramic material under vacuum and maintaining the melt at about 50* C above its melting point while unidirectionally cooling the melt by the passage of a coolant gas against the closed end of a heat exchanger, c. unidirectionally solidifying the ceramic material by decreasing the temperature while increasing the flow of coolant gas, and recovering the material.
 5. A method in accordance with claim 4 wherein the coolant gas is helium.
 6. A method in accordance with claim 4 wherein the ceramic material is selected from the group consisting of Al203, Y3Al5012, MgAl204, Al203/Zr02 and Al203/Y3Al5012 eutectics.
 7. A method in accordance with claim 6 wherein the ceramic material is Al203.
 8. A method in accordance with claim 7 wherein the premelt is seeded with its ''''a'''' axis parallel to the growth direction.
 9. A method in accordance with claim 7 wherein the premelt is seeded with its ''''c'''' axis 60* from the growth direction.
 10. A method in accordance with claim 6 wherein the ceramic material is Y3Al5012.
 11. A method in accordance with claim 10 wherein the premelt is seeded with (110) growth direction.
 12. A method in accordance with claim 6 wherein the ceramic material is MgAl204.
 13. A method in accordance with claim 6 wherein the ceramic material is Al304/Zr02 eutectic.
 14. A method in accordance with claim 6 wherein the ceramic material is Al203/Y3Al5012 eutectic. 